For now, the technique applies to quantitative thinking (doing math problems and calculating numbers) but the process could launch ways of reading other types of thoughts as well. The new research, published in Nature Communications, was the first to “read the minds” of patients who were not simply engaging in lab experiments but were going about their daily lives, albeit in a hospital where they were awaiting surgery. The patients, who had epilepsy, volunteered to have electrodes placed on the surface of their brains in order to determine what region was causing their seizures and whether the dysfunctional regions could be safely removed with surgery.

Because the electrodes remained in the patients’ brains for several days, Dr. Josef Parvizi, associate professor of neurology and neurological sciences at Stanford University, saw an opportunity to eavesdrop on their “thoughts” and link up specific brain activity with behaviors and reactions such as pain, eating, drinking, and talking. With the patients’ permission, Parvizi and his colleagues videotaped them as they interacted with friends, family and hospital staff and connected the participants’ experiences to their brain activity to get a more detailed and realistic understanding of brain function that static brain scans can’t provide.

“You are able to see how neurons within the human brain are working in a real life setting,” says Parvizi.

The three patients answered true or false questions that ranged from mathematical calculations to what scientists call episodic memory — such as, I had coffee at breakfast this morning. As with previous studies, the researchers found that a specific group of nerves in the brain was activated when the participants performed calculations. But when the team looked at the videos to correlate what the patients were doing during other periods of spiking activity in this region, they were delighted to find that the participants were discussing quantitative concepts.

One patient, for example, talked on the phone about being given “some more Vicodin” and the region— known as the intraparietal sulcus—sprang into action. She later mentioned a “ten to fifteen minute seizure,” and again, activity there spiked. Similarly, when the participants referred to more abstract numerical quantities such as ‘many,’ ‘bigger than,’ and ‘some more,’ the nerves also started buzzing with activity.

“This is exciting, and a little scary,” said Henry Greely, who chairs Stanford’s Center for Biomedical Ethics but was not involved with the research, in a statement. “It demonstrates, first, that we can see when someone’s dealing with numbers and, second, that we may conceivably someday be able to manipulate the brain to affect how someone deals with numbers.”

Even more tantalizing is the possibility of “reading” the passive thoughts of patients in comas, those who have experienced brain injury, or are paralyzed and unable to express themselves through speech. “I think it’s very promising,” says John Donaghue, professor of neuroscience at Brown University, commenting on the study. Donaghue has been working to develop implants to help paralyzed people move again— he demonstrated in several patients that implants in the motor cortex can help them to manipulate robotic arms just by thinking about the action. One woman, paralyzed for fifteen years, used such implants to will a robot arm to allow her to drink coffee independently for the first time since her stroke.

Donaghue says these studies hint that, at least in a crude way, it’s possible to read the mind’s ability to perform certain actions, and then tap into this system to generate specific movements at will. He and his colleagues have shown, for example, that monkeys with such implants can learn to move objects intentionally by thinking about it. “When the animals realize [they can do it], they get lazy and don’t want to go back to doing it [physically],” he says.

But applying this system to read thoughts and language will almost certainly be much more complicated. No one knows how the brain encodes specific concepts, words or numbers; the new research suggests where to look for numerical processing, but doesn’t reveal the content of that data— for example, which specific circuits are responsible for the number two, and which are charged with conveying the ‘greater than’ principle. “The only thing that we can tell is that they were thinking about numbers,” says Parvizi. In other words, they can tell you might be thinking about your winning lottery number— but not what the lucky number actually is.

“What’s going on in your mind right now involves the collective action of unbelievably large numbers of neurons,” says Donaghue, “The only way to truly understand that thought would be to have truly detailed measurements of all of it at once and we don’t the have tools to do that. Reading your email would probably be better.”

That doesn’t mean such neural eavesdropping won’t be possible. But, Parvizi said in a statement, “We’re still in early days with this. If this is a baseball game, we’re not even in the first inning. We just got a ticket to enter the stadium.”